Corrosion of metals and metallic surfaces in aqueous environments, such as water systems, is a significant problem, estimated by the National Association of Corrosion Engineers to cost approximately 3% of U.S. GDP. Corrosion inhibitors are commonly applied to aqueous systems to reduce corrosion damage. Precise dosing and control of corrosion inhibitors is required to achieve optimum performance.
Triazole compounds can be used to inhibit the corrosion of metals, such as copper, steel, and galvanized metal, in aqueous and non-aqueous environments. To function effectively in aqueous systems, the water contacting a metal surface must contain an appropriate concentration of the corrosion inhibitor. Maintaining the proper dosage can be problematic for several reasons. Industrial systems have water losses, either intentionally or due to leakage. The corrosion inhibitor must be replenished to account for these losses. Organic triazole compounds can be subject to losses due to biological degradation and must be replaced. Triazoles can be depleted as corrosion inhibitors by reaction with metal ions such as copper in solution.
In aqueous water systems, the concentration of triazole compounds is most commonly determined in the field using a colorimetric method involving the collection of a water sample, adding a series of reagents, and digesting the sample for several minutes using a strong UV light source, which produces a faint yellow color that can be correlated to the triazole concentration using a spectrophotometer or a handheld color comparator. Colorimetric methods are applied to discrete batch samples rather than being continuous. Moreover these types of methods would be difficult and expensive to automate, requiring sampling pumps, consumable reagents and pumps, and time delays during digestion stage. These factors make colorimetric assays of triazoles difficult to implement for in-line control.
In a well-equipped laboratory, the concentration of triazole compounds can also be determined by skilled chemists using high performance liquid chromatography (HPLC). HPLC involves injecting a known sample volume into a pumped eluent solution, which passes through a chromatography column and through a detector, generating a series of peaks on a chromatogram, which are evaluated and quantitated by the chemist. HPLC, however, may not be practical for in-line monitoring and control in industrial water systems.
UV fluorescence is another method for measuring and controlling the amount of organic azole corrosion inhibits. UV fluorescence offers the potential advantage of being reagent-free as the triazole compounds fluoresce to some extent when excited at the appropriate wavelength. UV fluorescence also offers the potential for rapid detection, which is more suitable for in-line process control.
However, the fluorescence signal of many triazole compounds, such as benzotriazole and tolyltriazole, are comparatively weak, making it difficult to detect their fluorescence from background fluorescence. A common method for overcoming this problem is to acidify the sample. The fluorescence of benzotriazole and tolyltriazole may be respectively increased by 6.4 and 10.6 times by acidifying the sample to a pH of 0.5.
Because most aqueous systems operate at neutral to slightly alkaline pH, acidification requires removal of a sample from the system and the addition of an acidic reagent to the sample to achieve the required sensitivity. In other words, the necessary acidic reagent typically cannot be added to the system. As a result, the requirement for an acidic reagent reduces the viability of this method for use in detecting, monitoring, and controlling amounts of the azole inhibitor throughout the system.
In addition to pH sensitivity, the fluorescent signal of the azoles also changes significantly in the presence of chlorine. Chlorine is the most common disinfectant used in aqueous systems. A stable control signal in the presence of chlorine or halogens would be desirable to achieve precise control in many aqueous systems.